Metal-to-metal bonding (also sometimes referred to as direct bonding) is a commonly used bonding method in the packaging of integrated circuits. In the direct bonding, the bond pads of two wafers or chips are bonded together without solder disposed in between. For example, the direct bonding may be a copper-to-copper bonding or a gold-to-gold bonding. The methods for performing the direct bonding include Thermal Compression Bonding (TCB, sometimes known as thermal compressive bonding). In a typical direct bonding process, the metal bumps of a device die are aligned to, and are placed against, the metal bumps of a package substrate. A pressure is applied to press the device die and the package substrate against each other. During the bonding process, the device die and the package substrate are also heated. With the pressure and the elevated temperature, the surface portions of the metal bumps of the device die and the package substrate inter-diffuse, so that bonds are formed.
The direct bonding typically requires large metal pads. The large metal pads, however, may incur severe dishing problem in the planarization of the metal pads. Furthermore, Misalignment between the two bonded package components may occur, and hence the contact area of the bonds may vary depending on the severity of the misalignment. The variation of the contact area results in the variation of the contact resistance, which in turn results in the variation in the performance of the resulting package. When the metal-to-metal bonding is used to bond integrated circuits that have high bond densities, due to the small sizes and the small pitches of the bond pads, the variation in the contact resistance is worsened.